1. FIELD OF THE INVENTION
The present invention relates to a viscosity damper for controlling vibrations caused by a motor such as a stepping motor.
2. DISCUSSION OF BACKGROUND
FIGS. 4 and 5 show a conventional viscosity damper described in, for instance, Japanese Examined Utility Model Publication No. 33314/1986. In FIGS. 4 and 5, reference numeral 1 designates the shaft of a stepping motor, numeral 2 designates a damper casing made of a resinous material such as polycarbonate which is attached to the shaft 1, numeral 3 designates an inertia body of a ring form made of metal such as lead, copper or iron, numeral 4 designates a viscous fluid filled in a space formed between the inner wall surface of the damper casing 2 and the inertia body 3. As the viscous fluid, silicon oil is generally used. Numeral 5 designates an introducing opening formed in one side of the damper casing 2 to introduce the viscous fluid 4, amnd a numeral 6 designates a cap for closing the introducing opening 5 after the viscous fluid is introduced. In the inertia body 3, a pair of annular grooves are formed in both side surfaces, and a plurality of through holes 8 are formed to communicate the annular grooves 7 at both side surfaces so that the viscous fluid 4 can be easily introduced in the damper casing 2.
Operation of the conventional viscosity damper will be described.
When the stepping motor shaft 1 is accelerated for rotation, the damper casing 2 is also rotated. In this case, the inertia body 3 is rotated independent of the damper casing 2 because it is not restricted by the casing 2. Accordingly, there takes place relative movement between the inner wall surface of the damper casing 2 and the front surface of the inertia body 3, whereby resistance of viscosity is producted by the viscous fluid 4 having a certain frictional coefficient of viscosity. The resistance of viscosity is against the relative movement between the damper casing 2 and the inertia body 3, whereby a damping force is produced to affect vibration controlling function. The viscosity of the viscous fluid 4 is so selected that a vibration controlling effect can optimally be applied to the moment of inertia of the inertia body 3. Generally, it is determined to be 100-2000 cst. In the conventional viscosity damper, the annular grooves 7 and through holes 8 are formed in the inertia body 3 so that the viscous fluid 4 is easily filled in the small space formed between the damper casing 2 and the inertia body 3.
Thus, in the conventional viscosity damper, the vibration controlling effect is primarily determined by the moment of inertia of the inertia body. Accordingly, when the inertia body having a large moment of inertia is used, it is very effective to vibrations having a large amplitude. On the other hand, when the inertia body having a small moment of inertia is used, it is very effective to vibrations having a small amplitude. However, the conventional viscosity damper could not provide effective vibration controlling effect irrespective of the magnitude of amplitude. Further, the conventional viscosity damper requires machining for the grooves and the through holes for the inertia body to thereby invite a high manufacturing cost.